The invention relates generally to wind power generation, and more particularly to techniques for controlling power ramp rates of a wind farm having multiple wind turbine generators.
A wind power generation system generally includes a wind farm having a plurality of wind turbine generators supplying power to a utility grid. Collective power output of the wind farm is greatly influenced by wind conditions on individual wind turbine generators. Utilities often have other power resources, such as thermal power plants to balance their electrical loads, thus accommodating variability in wind conditions during intermittent wind conditions. Thermal power plants may include, for example, coal and gas fired stations. Power fluctuation of wind farms due to gusty or low wind conditions is usually dealt with by adjusting power output of these thermal power plants to provide relatively constant overall power matching demands.
However, it is often difficult to change power output of thermal power plants instantaneously. Changing of power output may be also referred to as ramping. Thermal power generators desirably require a ramp rate that does not impose excessive thermal stresses, and that accommodate the natural lag times involved in heating and cooling the heat transfer components. As an example, coal-fired power stations may take over 12 hours to start from cold, and, even when hot, may take 2 to 3 hours to be ramped from 0-100% of their rated power. Ramping down of such thermal power generators may require even slower rates to minimize risk of damaging plant components. Wind conditions, on the other hand, may change drastically in a relatively shorter time span. It is, therefore, desirable to control power ramp rates of wind farms taking into consideration the maximum prescribed power ramp rates of such other power resources.
It is possible to limit power ramp rates of individual wind turbine generators at any level up to a maximum power value that is dependent on the current wind speed. This is achieved by curtailing a portion of the power output, so that the power ramp rate does not exceed a maximum desired ramp rate. However this limits the capture of wind energy and increases the effective cost of energy of the wind farm. Further, although power ramp rate control has been achieved to some degree by such techniques on individual generators, the problem has not been addressed at a wind farm level.
There is, hence, a need for a technique to control power ramp rates at a wind farm level within limits prescribed by transmission system operators, while allowing maximum capture of wind energy by individual wind turbine generators.